Field of the Invention (Technical Field)
The present invention relates to a technology developed for the highly selective extraction of univalent species from aqueous solutions. The technology is preferably expressed in terms of macroreticular (macroporous), strong base, ion exchange resin (IER) beads that have been specifically prepared for the complexation of univalent anions. The beads are preferably prepared using a functionalized monomer or in a conventional manner in terms of the order of steps, but with precisely controlled parameters and the use of a specifically tuned coordinator. The beads can also sense the degree of loading by being interrogated optically, resulting in a characteristic luminesce.
Background Art
Note that the following discussion may refer to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Precious metals are typically removed from ore using cyanide solutions that in the presence of atmospheric oxygen dissolve the metals to make soluble cyanide metal complexes. Some base metals, such as copper, mercury, lead and iron, are also dissolved and form cyanide complexes as well. The dissolved metals are then adsorbed on a material. The materials employed are selective in the adsorption process, but the current materials employed also adsorb base metal cyanide complexes. Many univalent ions have toxic properties or are economically useful. However, commercial ion exchange resins are more selective for ions with higher charges and are not suitable for the selective complexation of univalent species.
Carbon or activated charcoal is one of the materials used to adsorb precious metal cyanide complexes from aqueous solutions. Activated carbon is inexpensive to produce, absorbs dicyanoaurate readily, has a large capacity, and may be regenerated. Unfortunately, activated carbon also has a high affinity for mercury (II) tetracyanide, and under some conditions mercury (II) tetracyanide may actually displace dicyanoaurate from the activated carbon. Like dicyanoargentate, mercury (II) tetracyanide desorbs with dicyanoaurate when eluted from the activated carbon. Mercury (II) tetracyanide is also reduced to elemental mercury during the electrowinning process that isolates metallic gold. The mercury can be removed from the impure gold by thermal processes, but some is inevitably lost to the atmosphere. It would be preferable to use a sorbent that selectively removes gold and silver from the leachates to avoid environmental concerns. Furthermore, the elution process is not incomplete for activated carbon and some mercury remains on the activated carbon. During thermal reactivation of the activated carbon, the mercury is reduced to mercury metal that volatilizes and escapes into the atmosphere. The reactivation step is unavoidable as activated carbon also absorbs organic matter that fouls and reduces its sorption capacity. Thus the reuse of activated charcoal is limited and losses are significant.
Another method for removal of gold from cyanide leach solutions involves anion exchange resins. Both weak base and strong base anion exchange resins have been employed. Ion exchange should also allow better recycling of cyanide. Certain specialty resins have been applied to sequestration of gold and silver. The issue with existing IERs is a lack of specificity, due to the method used to prepare anion exchange resins. High capacity is sought by applying a poorly controlled high degree of functionalization. This results in close proximity of binding sites resulting in a higher affinity for species with multiple charges. This process also makes these resins unlikely to have selectivity for any univalent species.
Ion imprinting can be used to manufacture beads with a higher selectivity than commercial beads. However, the coordination sites are typically aggregated in this process, which increases cooperative effects leading to lower selectivity for target ions and increasing selectivity for polyvalent ions. In addition, the manufacturing method is expensive and hazardous, requiring the use of precious metals and cyanide. The resulting beads always have residual precious metal cyanide complexes as well.
Selectivity of conventional strong base anion exchange resins is usually explained in terms of the size of the hydrated ion, the charge on the ion and sieving effects (large ions). Specific associations with the ionogenic site are usually overwhelmed by physical parameters. Thus the counter ion having a higher valence, having the smallest hydrated volume (unless the crosslinking is so high that only an unsolvated ion may penetrate), of greater polarizability, or that is least involved in complexation will be preferred. These properties are a result of the degree of functionalization and the hydrophobicity of the binding site.
Anion association can also be understood as a function of the quaternizing agent (hydrophobicity) and the loading of the ionogenic sites. The preference for ions of higher valence can be addressed by controlling the degree of functionalization and the distribution of sites. Conventional strong base anion exchange resins are functionalized as heavily as possible using a hydrophilic quaternizing agent. Using a less hydrophilic quaternizing agent can change the hydrophilicity. This combined with higher degrees of crosslinking and greater sieving effects are particularly important when building selectivity for complex species.